Rework std_lib test to make it more realistic

compile-tests/std_lib.rs

 #[no_mangle]
-extern "C" fn root(a: Vec<String>,
-                   b: Option<i32>,
-                   c: Result<i32, String>)
+extern "C" fn root(a: &Vec<String>,
+                   b: &Option<i32>,
+                   c: &Result<i32, String>)
 { }